What you don’t have the right to ignore about LEDs

What you don’t have the right to ignore about LEDs

My goal here is not to bombard you with frightening physics equations and terms, you can find them by yourself in many sites. You can also read the Wikipedia 10 pages article (http://en.wikipedia.org/wiki/LED). I only want to compile some essential concepts that will allow Newcomers to understand the “basics of basics”.
Here is a small list of what you don’t have the right to ignore. The real world is of course a little bit more complex than that, but this list is a good starting point:

L.E.D. is for Light Emitting Diode. It’s a small die that emits light when crossed by a current. This die needs to be electronically connected and mechanically protected, and that’s why it’s usually sold as an encapsulated device. By extension, this encapsulated device is called LED too.

about the die:
- usually square, between 250 and 1000 microns. The larger sizes are called High Brightness (HB)
- there are roughly 2 families of materials, one for the higher part of the spectrum (yellow to IR), the other one for the lower part (UV to green). Those 2 families differ in their electrical behavior.

electrical facts:
Like usual electronic diodes, the current can travel only in one direction inside the LED, and is blocked in the other direction. But, even in the right direction, there is a voltage threshold:
- below the voltage threshold, no light is emitted
- above the voltage threshold, the LED emits light, and the optical power goes with the current (and NOT with the voltage)

optical facts:
- the spectrum emitted by a die is close to a Gaussian curve, FWHM is between 25 and 40 nm
- you can find LEDs in numerous wavelengths and optical powers (but not all) from the UV to the near IR. There are even some LEDs in the 200+ nm region
- the die itself emits a Lambertian beam (half-sphere emission)
- the encapsulation can act as a lens, and the emitted beam is narrowed
- a “white LED” is either an encapsulated LED composed of 3 RGB (Red Green Blue) dice, or a blue / UV LED covered with phosphor.

What did I forget in this list?